Whooping cough, or pertussis, is a severe, highly contagious respiratory disease of infants and young children caused by infection with Bordetella pertussis. Owing to the many virulence factors associated with this organism, the pathogenesis of the disease is still not fully understood; however, it is generally recognized that the major systemic effects are caused by pertussis toxin (PT). This material exhibits a wide range of biological activities as illustrated by such alternative names as lymphocytosis-promoting factor, histamine-sensitizing factor and islet-activating protein. Many of these effects are associated with its biochemical function as an adenosine diphosphate (ADP)-ribosyltransferase. ADP- ribosylation of certain acceptor guanosine triphosphate-binding proteins leads to a loss of control over a variety of metabolic pathways mediated by cyclic adenosine monophosphate and by phospholipase C. In the absence of a protein acceptor, PT also catalyses the hydrolysis of nicotinamide adenine dinucleotide (AND glycohydrolase activity).
Conventional killed whole-cell pertussis vaccines contain a mixture of antigens and there has been a great deal of work towards the development of a defined acellular vaccine comprising specific protective antigens. PT is the most significant protective antigen. Other antigens under consideration are agglutinogens and filamentous hemagglutinin (FHA).
Normally PT and other antigens are chemically inactivated, or toxoided, using agents such as formaldehyde, glutaraldehyde or hydrogen peroxide. This approach has the serious disadvantage that a delicate balance must be sought between too much and too little chemical modification. If the treatment is insufficient, the vaccine may retain residual toxicity owing to the presence of a small proportion of unchanged virulence factors including PT. If the treatment is too excessive, the vaccine may lose potency because its native immunogenic determinants are masked or destroyed. This problem is of particular concern in the case of PT, since the catalytic subunit is comparatively difficult to inactivate by aldehydes. The possible residual toxicity or reversion of toxoided whole-cell pertussis vaccines has been questioned for many years, and it has suggested that in rare cases the vaccine might cause major neurological damage. All pertussis vaccines that are in use at present, or in the trial stages, depend on the inactivation of the antigens by chemical means, which introduces the problems previously mentioned. It is obvious that if an inactivated vaccine could be designed without resorting to the toxoiding process, but preserving the native structure of immunogenic and protective epitopes, an additional degree of safety and efficacy would be added. For these reasons the inventors have genetically manipulated the gene coding for PTTOX-, and constructed strains of pertussis that secrete non-toxic PT analogues.
In its structural organization, PT belongs to the family of ADP-ribosyltransferase bacterial toxins, which also includes diphtheria toxin, Pseudomonas aeruginosa exotoxin A, cholera toxin and Escherichia coli heat labile toxin. Accordingly, it consists of two functional moieties: an A portion, which carries the enzymic activity, and a B portion, which binds to the host cell and permits translocation of the A portion to its site of action. In PT, the A portion is a discrete subunit, commonly denoted S1. The B portion is a non-covalent oligomer of five polypeptides arranged as two dimers, comprising subunits S2 plus S4 and subunits S3 plus S4 respectively, held together by a joining subunit S5.
The amino acid sequence of the S1 subunit reveals several features of interest. There are only two cysteine residues which form an intrachain disulphide bond; however, it is known that for enzymic activity the toxin must be reduced (Moss et al., J.Biol.Chem. 258, 11879, [1983]), indicating the importance of these residues. There are two tryptophans in S1, and it has been suggested that tryptophan residues are close to the AND binding sites of diphtheria toxin and P. aeruginosa exotoxin A. Two conserved regions in S1 are also found in the amino acid sequences of cholera toxin and E. coli heat labile toxin (Locht & Keith, Science, 232, 1258, [1986]). In addition the AND active sites of diphtheria toxin and P. aeruginosa exotoxin A have been shown to contain a glutamic acid residue (Carrol & Collier, Proc. Nat. Acad. Sci., U.S.A., 81, 3307, [1984]; Carroll & Collier, J.Biol.Chem., 262, 8707, [1987]).
As noted above, the B portion of PT mediates its binding to cellular receptors and contains two dimers. Whether each of these dimers bears a binding site remains controversial. However, the S2 and S3 subunits are similar in amino acid sequence and binding studies have indicated that lysine and/or tyrosine residues of S3 in particular are implicated in the interaction of the toxin with its receptor. (Nogimori et al., Biochem., 25, 1355, [1986]; Armstrong & Peppler, Infect. Immun., 55, 1294, [1987]).
Site-directed mutagenesis of diphtheria toxin and P. aeruginosa exotoxin A at the AND-interacting glutamic acid residues has led to significant reduction in ADP-ribosyltransferase activity (Tweten et al., J.Biol.Chem., 260, 10392, [1985]; Douglas & Collier, J.Bacteriol., 169, 4967, [1987]). Complete truncated forms of S1 and S2 have been expressed in E. coli (Locht et al. Infect. Immun., 55, 2546, [1987]). Mutations of the TOX operon generated by transposon insertion, gene truncation or linker insertion have been introduced by allelic exchange into the chromosome of B. pertussis (Black et al., Ann. Sclavo, 175, [1986]; Black & Falkow, Infect. Immun., 55, 2465, [1987]). However, the biological and immunoprotective properties of fully-assembled recombinant holotoxins specifically detoxified by site-directed mutagenesis of functional amino acid residues have not been reported. The generation of such PT analogues for inclusion in a safe and efficacious pertussis vaccine is the subject of this invention
In testing for the efficacy and toxicity of materials that could be candidates for a protective vaccine, there are a number of in vivo and in vitro assays available. The standard test for potency is the mouse protection test, which involves intra- cerebral challenge with live B. pertussis. Newer vaccine tests measure the production of protective antibodies. A common toxicity test is the CHO (Chinese hamster ovary) cell clustering assay, which reflects both the ADP-ribosyltransferase and binding ability of the toxin (Burns et al., Infect. Immun., 55, 24, [1987]). A direct test of the enzymic activity of PT is the ADP-ribosylation of bovine transducin (Walkins et al., J. Biol. Chem., 260, 13478, [1985]).